This year's megadeal-driven boom in mergers and acquisitions has largely benefited a small group of elite law firms.

The top five law firms, ranked by deal dollar values, had more than 75% of the market in the first half, the highest level in five years, according to Dealogic. Skadden, Arps, Slate, Meagher & Flom LLP was atop the heap, with a hand in one of every five dollars committed to M&A—the highest market share captured...